Definition of ADMIN$ ADMIN in The Network Encyclopedia.

What is ADMIN$

A special administrative share created during installation on computers running Microsoft Windows NT and Windows 2000 and used for remote administration of the computer. The path of this share is always the path to the %SystemRoot% directory (usually C:\Winnt). The Administrators group is assigned full control permissions on the ADMIN$ share on a Windows NT system. This allows administrators to access the share and remotely administer the system without needing to know where the system files are stored. Only the Administrators group has access to the ADMIN$ share.

Admin stands for Administrator and Admin$ is an administrative share to the administrators group.